Abstract
Abstract. It is projected that forest disturbances, such as insect outbreaks, will have an increasingly negative impact on forests with a warmer climate. These disturbance events can have a substantial impact on forests' ability to absorb atmospheric CO2, and may even turn forests from carbon sinks into carbon sources; hence, it is important to develop methods both to monitor forest disturbances and to quantify the impact of these disturbance events on the carbon balance. In this study we present a method to monitor insect-induced defoliation in a subarctic birch forest in northern Sweden, and to quantify the impact of these outbreaks on gross primary productivity (GPP). Since frequent cloud cover in the study area requires data with high temporal resolution and limits the use of finer spatial resolution sensors such as Landsat, defoliation was mapped with remote sensing data from the MODIS sensor with 250 m × 250 m spatial resolution. The impact on GPP was estimated with a light use efficiency (LUE) model that was calibrated with GPP data obtained from eddy covariance (EC) measurements from 5 years with undisturbed birch forest and 1 year with insect-induced defoliation. Two methods were applied to estimate the impact on GPP: (1) applying a GPP reduction factor derived from EC measured GPP to estimate GPP loss, and (2) running a LUE model for both undisturbed and defoliated forest and deriving the differences in modelled GPP. In the study area of 100 km2 the results suggested a substantial setback to the carbon uptake: an average decrease in regional GPP over the three outbreak years (2004, 2012, and 2013) was estimated to 15 ± 5 Gg C yr−1, compared to the mean regional GPP of 40 ± 12 Gg C yr−1 for the 5 years without defoliation, i.e. 38 %. In the most severe outbreak year (2012), 76 % of the birch forests were defoliated, and annual regional GPP was merely 50 % of GPP for years without disturbances. The study has generated valuable data on GPP reduction, and demonstrates a potential for mapping insect disturbance impact over extended areas.
Highlights
It is estimated that forests account for half of the global terrestrial net primary productivity and act as important sinks of atmospheric CO2 (Bonan, 2008)
These insect outbreaks strongly influence the birch forests (Ammunét et al, 2015): severe defoliation events may result in stem mortality, requiring decades of recovery (e.g. Tenow, 1996; Tenow and Bylund, 2000; Jepsen et al, 2013), and understorey vegetation can shift into more grass-dominated communities (Karlsen et al, 2013; Jepsen et al, 2013)
An ordinary least squares (OLS) regression equation was calculated with NDVIDL values ≥ 0.4 to model the relationship between fAPAR8day and NDVIDL
Summary
It is estimated that forests account for half of the global terrestrial net primary productivity and act as important sinks of atmospheric CO2 (Bonan, 2008). One important forest disturbance agent is insects; it is projected that the temporal and spatial dynamics, as well as the intensities and ranges of insect herbivore outbreaks, will be influenced by global warming (Vanhanen et al, 2007; Battisti 2008; Jepsen et al, 2008; Netherer and Schopf, 2010). These insect outbreaks can severely disturb forest ecosystems, and have a strong impact on carbon dynamics (Kurz et al, 2008a; Jepsen et al, 2009; Heliasz et al, 2011). Quantitative effects of insect outbreaks on the carbon balance are, not well known
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